1. Field of the Invention
The present invention relates to a reflective display device including a retroreflective layer.
2. Description of the Related Art
A reflective liquid crystal display device for conducting a display operation by utilizing surrounding light as its light source has been known in the art. Unlike a transmissive liquid crystal display device, the reflective liquid crystal display device needs no backlight, thus saving the power for light source and allowing the user to carry a smaller battery. Also, the space to be left for the backlight in a transmissive device or the weight of the device itself can be saved. For these reasons, the reflective liquid crystal display device is effectively applicable to various types of electronic devices that should be as lightweight and as thin as possible.
A technique of combining a scattering type liquid crystal display mode and a retroreflector is one of known measures to improve the display quality of a reflective liquid crystal display device. Such a technique is disclosed in Japanese Patent Applications Laid-Open Publications Nos. 5-107538, 2000-19490, 2002-107519 and 11-15415, for example.
Hereinafter, the operation principle of a display device that adopts such a technique will be described with reference to FIGS. 1A and 1B, which schematically illustrate the black and white display modes of the display device.
As shown in FIG. 1A, if a liquid crystal layer 1 is controlled to exhibit a transmitting state, an incoming light ray 3, which has been emitted from a light source 5 outside of the display device, is transmitted through the liquid crystal layer 1 and then reflected back by a retroreflector 2 toward its light source 5 as pointed by the arrow 4b. Thus, the light ray 3 that has been emitted from the light source 5 does not reach the eyes of a viewer 6. In such a state, the image reaching the eyes of the viewer 6 from this display device is the image of his or her own eyes. In this manner, the “black” display mode is realized.
On the other hand, if the liquid crystal layer 1 is controlled to exhibit a scattering state, the incoming light ray 3 that has been emitted from the light source 5 is scattered by the liquid crystal layer 1 as shown in FIG. 1B. Specifically, if the liquid crystal layer 1 is a forward scattering liquid crystal layer, most of the incoming light ray 3 is scattered forward by the liquid crystal layer 1 and then reflected back by the retroreflector 2 toward the viewer 6 through the liquid crystal layer 1 in the scattering state (as pointed by the arrows 4w). In this case, since the retroreflectivity of the retroreflector 2 is disturbed by the scattering caused by the liquid crystal layer 1, the incoming light ray 3 does not return to its light source. In the meantime, another portion of the incoming light ray 3 is scattered backward by the liquid crystal layer 1 and directed toward the viewer 6 (not shown). In this case, that portion of the light directed toward the viewer 6 reaches his or her eyes, thus realizing a “white” display mode. According to this operation principle, not just the backscattering but also forward scattering of the liquid crystal layer 1 can be used effectively. As a result, a brighter “white” display is achieved.
By conducting a display operation based on this operation principle, a monochrome display is realized without using any polarizer. Consequently, a high-brightness reflective liquid crystal display device, of which the optical efficiency is not decreased by the use of polarizers, is realized.
As the retroreflector 2 shown in FIGS. 1A and 1B, a two-dimensional array of unit structures, such as a micro-sphere array, a microlens array or a corner cube array, may be used. Among these various types of arrays, the “corner cube array” is a two-dimensional arrangement of corner cubes, each defined by three planes that are opposed substantially perpendicularly to each other, on a certain “virtual plane”. The “virtual plane” is typically a plane that is defined parallel to the surface of the display panel of a display device. A light ray that has entered a corner cube is ideally reflected back toward its source by the three planes that form the corner cube. FIGS. 2A and 2B are respectively a plan view and a perspective view illustrating the configuration of a corner cube array. The corner cube array shown in FIGS. 2A and 2B is a cubic corner cube array in which a number of corner cubes, each being defined by three square planes that are opposed perpendicularly to each other, are arranged two-dimensionally.
A corner cube array may have a high retroreflectivity. That is why by using a corner cube array, the contrast ratio can be increased on the display screen of a reflective display device. To further increase the contrast ratio on the screen of a reflective display device that uses a corner cube array, Japanese Patent Application Laid-Open Publication No. 2002-107519 suggests that a corner cube array consisting of corner cubes of a reduced size be used as a retroreflector. A corner cube array consisting of corner cubes of such a reduced size (e.g., with an arrangement pitch of 5 mm or less) will be referred to herein as a “micro corner cube array (MCCA)”. Also, the arrangement pitch of corner cubes in an MCCA is identified herein by Pcc (i.e., the shortest distance between two adjacent vertices) as shown in FIG. 2A.
Next, a specific configuration for a reflective display device that uses an MCCA as a retroreflector will be described.
A reflective display device with an MCCA may be formed by arranging the MCCA outside of a display panel such that the MCCA is located on the opposite side (i.e., the non-viewer side) of the display panel. Such an arrangement in which an MCCA is attached to the non-viewer side of a display panel (which will be referred to herein as an “MCCA attached structure”) is disclosed in Japanese Patent Application Laid-Open Publication No. 11-15415, for example. As used herein, the “display panel” refers to a panel in which a modulating layer such as a liquid crystal layer and a voltage application means for applying a voltage to the modulating layer are sandwiched between two opposed substrates. Of these two opposed substrates, the one substrate to face the viewer will be referred to, herein as a “front substrate” and the other substrate not to face the viewer a “rear substrate”. In the MCCA attached structure, the MCCA is arranged behind the rear substrate.
Meanwhile, a reflective display device with a structure in which an MCCA is arranged between the two substrates of a display panel (which will be referred to herein as an “MCCA embedded structure”) was also proposed. For example, Japanese Patent Application Laid-Open Publication No. 2002-107519 mentioned above discloses a structure in which a retroreflector is arranged between the modulating layer and the rear substrate of a display panel.
In a reflective display device that uses an MCCA, the black display may sometimes have a slightly decreased contrast ratio due to the leakage of light and turn slightly lightened black (which is called a “dark-state leakage”) or white and black may sometimes be inverted in a grayscale tone display mode (which is called a “grayscale inversion”) due to the shape and plane accuracy of the MCCA or according to the direction in which light has entered the MCCA. The present inventors analyzed these problems extensively. And the results of our analysis will be described with reference to the accompanying drawings.
In the following example, MCCA Nos. 1, 2 and 3 with mutually different shapes will be described. FIGS. 3A, 3B and 3C are top views illustrating the unit structures (i.e., corner cubes) of MCCA Nos. 1, 2 and 3, respectively.
FIG. 3A shows one of the corner cubes that form MCCA No. 1. This corner cube has three rectangular isosceles triangular planes that are opposed substantially perpendicularly to each other, and is illustrated as an equilateral triangle 7 consisting of three isosceles triangles on this top view. In MCCA No. 1, a lot of corner cubes like this are arranged on a virtual plane. FIG. 3B shows seven of a huge number of corner cubes that form MCCA No. 2. Each of the corner cubes forming MCCA No. 2 is illustrated as a regular hexagon 8, of which the center is defined by a bottom point, on this top view. FIG. 3C shows six of a huge number of corner cubes that form MCCA No. 3. Each of the corner cubes forming MCCA No. 3 is illustrated as a rectangle 9, of which the center is defined by a bottom point, on this top view.
First, a situation where the corner cubes that form an MCCA are not represented by a point symmetric pattern, of which the center of symmetry is defined by a bottom point, on a top view will be described. For example, the corner cube shown in FIG. 3A is represented by the equilateral triangle 7 on its top view but the equilateral triangle 7 is not point symmetric. Thus, in an MCCA consisting of such corner cubes, even a light ray that has been incident perpendicularly to the MCCA may not be reflected by all three planes that form a corner cube depending on its point of incidence on the corner cube. It should be noted that the MCCA has corner cubes that are arranged two-dimensionally on a virtual plane as described above. Thus, the phrase “perpendicularly to an MCCA (or a corner cube array)” means herein “perpendicularly to the virtual plane of the MCCA. Generally speaking, if something enters perpendicularly to an MCCA, it will impinge perpendicularly onto the surface of the display screen of a display device including the MCCA.
As shown in FIG. 3A, a light ray that has been incident perpendicularly onto an MCCA 1 at a point a is reflected from the points a, b and c in this order. That is to say, the light ray is sequentially reflected by the three planes of a single corner cube back toward its source. However, another light ray that has been incident perpendicularly onto the MCCA 1 at another point d is reflected from the points d and e in this order and then leaves this corner cube. In other words, the light ray that has been incident on the point d is reflected by only two of the three planes of the corner cube and therefore, is not retroreflected but goes in a different direction. Stated otherwise, a light ray that has been incident from that different direction onto the point e will be reflected from the point d and then leave the corner cube perpendicularly to the MCCA 1.
As described above, when a light ray is incident perpendicularly onto an MCCA, the light ray will be retroreflected just as intended if the light ray enters predetermined areas of the three planes of a single corner cube. In the corner cube shown in FIG. 3A, the predetermined areas are represented by a regular hexagon 7′, of which the center is defined by the bottom point on its top view. Meanwhile, a light ray that has been incident onto other areas of the three planes of the corner cube will be reflected in a different direction, not the direction the light ray has come from. This means that if a light ray has entered a corner cube from the former direction, then the light ray will be reflected away perpendicularly to the MCCA. That is why in a reflective display device using such an MCCA, even if the viewer faces its screen squarely, part of the external light to be used for display purposes in a black display mode will enter his or her eyes, thus causing the dark-state leakage or grayscale inversion mentioned above.
Next, a situation where the corner cubes that form an MCCA are represented by a point symmetric pattern, of which the center of symmetry is defined by the bottom point, on its top view will be described. For example, each of the corner cubes shown in FIGS. 3B and 3C is represented by a point symmetric pattern, of which the center of symmetry is defined by the bottom point, on its top view (i.e., a regular hexagon 8 or a rectangle 9). Likewise, in the cubic corner cube array that has already been described with reference to FIGS. 2A and 2B, each corner cube is represented by a regular hexagon, of which the center of symmetry is also defined by the bottom point, on its top view.
Any light ray that has been incident perpendicularly onto an MCCA consisting of such corner cubes will always be reflected back toward its source by the three planes of one of its corner cubes, including the point of incidence, no matter at which area of the corner cube the point of incidence is located. Consequently, if the viewer squarely faces the screen of a display device including such an MCCA, no dark-state leakage or grayscale inversion should occur as a matter of principle. However, even in a reflective display device including such an MCCA, if a light ray is incident non-perpendicularly onto the MCCA, part of the light ray will not be reflected back toward its source as will be described in detail later. That is why the dark-state leakage or grayscale inversion may still happen depending on the direction in which the viewer is watching the screen of that display device.
Furthermore, no matter what planar pattern each of the corner cubes of an MCCA has, it is extremely difficult to define the corner cubes with high plane accuracy, particularly when the corner cubes are arranged at a very small pitch in the MCCA. Thus, each corner cube will actually have some errors in its “normal angle” or its degree of planarity. As used herein, the “normal angle” refers to the angle formed between a normal to some plane of a corner cube and a virtual plane. Therefore, if the normal angle has some error, then the angle defined by a normal to one plane of a corner cube with respect to the virtual plane is not an ideal one, and the angle formed by the three planes of the corner cube is not equal to 90 degrees, either. On the other hand, the error of the planarity refers to partial or entire warp of some plane of a corner cube and to rounding of the peak or bottom point of a corner cube. Due to those errors, the retroreflectivity of the MCCA decreases. As a result, even if the viewer is facing the screen squarely, dark-state leakage or grayscale inversion may still happen.
Consequently, it is difficult to eliminate such dark-state leakage or grayscale inversion and get excellent display quality realized by using a conventional reflective display device with an MCCA.